I-131 meta iodobenzylguanidine is a pharmaceutical that locates in pheochromocytomas and carcinoid tumors. In this project, pheochromocytoma and carcinoid tumors are detected by radioscintigraphy, using 131I- MIBG. The test is used to pre-operatively evaluate benign pheochromocytomas to rule out or locate multiple tumors, to aid in location of malignant pheochromocytomas before and during therapy, and to help locate ACTH producing carcinoid tumors. Overall, we have studied fourteen patients with malignant pheochromocytoma. Ten of these had positive scans which identified metastases. Four patients were negative. Despite the negative scans, the tumors in this latter group were large. Thirteen patients had benign pheochromocytomas. Of this group, the MIBG scans were positive in ten. In one patient with an ACTH producing carcinoid tumor, the MIBG images were normal and did not localize the source of the ACTH. It is apparent that MIBG adrenal scanning is useful preoperatively in ruling out multiple or ectopic pheochromocytomas. In addition, MIBG scanning can be used to locate recurrent pheochromocytomas when previous surgery or surgical clips interfere with CT or MRI images. The utility of MIBG scanning in patients receiving chemotherapy to define the extent of tumor has not been established. The ongoing study compares the MIBG scan technique with CT and MRI scanning for locating these tumors.